Tectonic control and rapid ascent of Patagonian lavas

HALLER MJ

Lajosszentmize

Conferencia; Second International Maar Conference; 2004

The occurrence of Crater Basalt Volcanic Field (CBVF) in Northern Patagonia is related to a very significant tectonic feature, the Gastre Fault System (GFS).  This fracture, a continental-scale lineament, extends from the Pacific coast at 38° S to the Atlantic Ocean, at 45° S, separating two different geological domains.  The length and long-term existence of GFS suggest that this main structural feature propagates deep in the crust. In the CBFV area, the NW trending fractures delimit the 30 km wide Gastre trench.  Most of the CBVF was poured out within the trench.  Other eruptive centers are located in the South-West limit of the graben. Kinematics analysis reveals a sinistral transcurrent fault behavior for the GFS, allowing to propose the existence of a transtensional trench in this segment of the fracture system.  Significant movements occurred during upper Cretaceous, with reactivations in Late Neogene and Quaternary.  Presuming a simple shear deformation, CBVF eruptive centers are aligned following the tension and synthetic shear fault directions of an ideal deformation ellipse. Although the synthetic shear faults do not constitute a primary tensional system, they are crust weakness zone that eventually can behave as tension fractures during an episode of compressional relief. The calculated crystallization temperatures close to 1200° C for the CBVF lavas are also found at the base of the lithosphere. The lack of conspicuous zonation in olivines and the lack of exsolution textures in pyroxenes are evidences of rapid cooling.  On the other hand, the lack of crust contamination and differentiation indicators in the geochemistry of CBVF basalts can mean that there was no magmatic chamber related to this volcanism.  The rapid ascent through the crust could be favored by the existence of deep fractures like GFS that would reach the base of the lithosphere inducting tectonic decompression and melting.